Side impact events in vehicles have been identified as one of the top priorities for both research and regulation with government requirements continuing to become more stringent. These additional requirements make designs for door trim systems more challenging because they may impact the door trim at the pelvic bolster. It is known in vehicles to provide a pelvic bolster composed of polymerized foam, such as polyurethane (PU) and expanded polypropylene (EPP). While providing certain attractive features, both materials have limitations. Specifically, in the case of polyurethane, the material cost is relatively high. This becomes of increasing concern particularly as pelvic safety zones are increased in size. In the case of expanded polypropylene, while the cost of this foam tends to be lower than the cost of polyurethane, this material has other limitations. First, expanded polypropylene tends not to use modifiers, so there may be some temperature dependence. Second, expanded polypropylene tends to be heat staked, thus reducing the volume available for energy absorption.
In addition, both polyurethane and expanded polypropylene are inherently separate from the door trim substrate, which adds to manufacturing operations, costs and complexity. Some efforts are being made to provide pelvic bolsters, separate from the door trim substrate, formed from a material other than foam, such as injection molding, but these efforts are also unsatisfactory because of manufacturing and assembly costs.
Accordingly, as in so many areas of vehicle technology, there is room in the art of vehicle door design for an alternative configuration to known pelvic bolster structures which provides effective protection that can be adapted to a variety of shapes while maintaining relatively low manufacturing and assembly costs.